Field of the Disclosure
The present invention relates in general to abrasive wheels and, in particular, to a system, method and apparatus for abrasive articles having improved fracture properties and grinding performance.
Description of the Related Art
Phenolic-based grinding wheels are made by sequentially charging into a mold layers of an abrasive mix and fiber glass web reinforcements, consolidating the components with pressure and then subsequently curing in an oven at elevated temperatures. In some cases the composition of the abrasive mix in the multilayered wheels may be different. These compositional differences in the layers are used to provide advantages in either or both performance and economics. Both single and double layered wheel compositions are conducive to high through-put manufacturing processes such as the shuttle box presses. Incorporation of compositional variations within the core of the wheel could provide additional economic and strength advantages. The process for incorporating a core having a composition other than that of the grinding zone requires additional and specialized equipment such as a containment ring of specific diameter and height that allows filling of the core with a distinctively different abrasive mix composition. Once the core is filled to the desired level with the abrasive mix, the containment ring is carefully removed so as not to perturb the two adjacent compositions. This operation is tricky and not conducive to high throughput wheel making.
Phenolic-based resins used to manufacture grinding wheels are inherently brittle materials that are subject to failure due to the probability of defects within the part. Reinforcements are therefore used in most wheels to preclude brittle and catastrophic failure.
One such reinforcement is a fiber glass web or fabric of various weights and styles. The webs are designed to improve the radial strength and prevent the explosive release of wheel fragments in the event that the wheel breaks during use. The web comprises a plurality of individual yarns or strands woven into a 0°/90° open structured fabric. The fabric is dipped in a phenolic resin to form a coating and subsequently dried or cured. Once the coating is cured to the desired level, the web is wound into a roll for easy storage until needed. The final step in preparing the web for use in the wheel is unwinding the roll and cutting individual circles having the desired dimensions. Significant waste is generated from cutting the appropriately shaped discs used to reinforce the wheel from the roll of web. The process is labor and time intensive, generates significant waste and is therefore expensive. Additionally, these fiber webs have a detrimental effect on grinding performance.
Chopped strand fibers also have been used to reinforce resin-based grinding wheels having a thick cross-sectional area. The chopped strand fibers are typically 3 to 4 mm in length and include a plurality of filaments. The number of filaments can vary depending on the manufacturing process but typically consists of 400 to 6000 filaments per bundle. The filaments are held together by an adhesive known as a sizing, binder, or coating that should ultimately be compatible with the resin matrix. The sizing comprises less than 2 wt % of the reinforcement. The amount of sizing or coating is limited by the current manufacturing processes used to make direct sized yarn or chopped strand products. One example of a chopped strand fiber is referred to as 174, available from Owens Corning.
Incorporation of chopped strand fibers into a dry grinding wheel mix is generally accomplished by blending the chopped strand fibers, resin, fillers, and abrasive particles for a specified time and then molding, curing, or otherwise processing the mix into a finished grinding wheel. High levels of chopped strands fibers in these mixes are inherently difficult to transfer into the mold and level or spread due to fiber bridging effects. Additionally, as the fiber bundles are dispersed into filaments, the bulk density decreases (volume increases) and mold filling with the correct amount of mix becomes more difficult. Chopped fibers in wheels having thin cross sections are not used because of these inherent difficulties associated transferring the mix and filling the mold.
Chopped strand fiber reinforced wheels typically suffer from a lower strength, presumably due to incomplete dispersal of the filaments within the chopped strand fiber bundle poor adhesion with the matrix resin, fiber length degradation, or a combination thereof.
There is therefore a need to be able to make multi-compositional zoned wheels with improved reinforcements using the shuttle-box process that can provide higher strength and higher fracture toughness without compromising grinding performance.